Z-source inverters with enhanced voltage boost for renewable energy systems
Date of Issue2012
School of Electrical and Electronic Engineering
Worldwide economic growth has initiated a strong demand for energy, which unfortunately cannot be met indefinitely by fossil fuels. That motivates the development of renewable energy systems like wind, solar and tidal farms, which by nature are non-polluting and abundant. Outputs from these renewable sources are also mostly electrical, which by far is the most efficient form of energy for transmission and distribution over long distance. Renewable energy is therefore a promising green alternative even though it is unlikely to replace fossil fuels any sooner. Although renewable energy is mostly electrical, connecting renewable sources to the utility mains or localized smart grids is not a trivial task, frequently complicated by climatic variations. Energy conditioning is therefore almost always needed, and is usually achieved by placing a power electronic conditioner between each renewable source and the grid. A typical power conditioner would consist of a power flow controller and a power converter interfaced by sensors and semiconductor driving circuits. The power controller would add features like maximum power point tracking, grid synchronization and fast dynamics to the system, which are presently well established. Upon receiving the command from the controller, the power converter realizes the physical conversion of magnitude, frequency and phase of the electrical quantities before smooth grid connection can be effected. A frequent feature demanded by the conversion is voltage boosting, which presently is met by introducing an elementary dc-dc boost converter to the system. This certainly is a straightforward and reliable extension, which presently might not have further scope for improvement. A more recent recommendation is to use the Z-source dc-ac inverters either alone or accompanied by a front-end rectifier if ac inputs are encountered like in wind and tidal generation. Adding a rectifier will strictly not modify the basic Z-source energy inversion concept, and is therefore not specifically addressed in this thesis. Unlike traditional inverters whose output voltages can only be stepped down, Z-source inverters form a new class of impedance-source inverters whose outputs can both be stepped up and down. This flexibility is accompanied by further improvement in robustness since accidental turning on of two switches from the same phase will no longer cause damages. Dead-time protecting against such shoot-through condition is therefore not needed. Instead, shoot-through condition is intentionally used by the Z-source inverters for safe voltage boosting without compromising their usual voltage buck ability. Z-source inverters are therefore promising alternatives for renewable generation, and have earlier been tested for solar and wind generation. Despite these improvements, Z-source inverters are presently known to be burdened by low modulation ratios at high gains, whose consequential effects are high component stresses and poor spectral performances. To avoid these shortcomings, the intention here is to formulate a few new Z-source inverters whose overall modulation ratios can be raised without limiting gains. Their component stresses will be reduced greatly, while voltage boost, reliability and waveform quality will be enhanced strongly. These advantages have already been proven in theory, simulation and experiment, and would definitely raise the attractiveness of Z-source inverters as the power stages of renewable systems.
DRNTU::Engineering::Electrical and electronic engineering::Power electronics